Nanoscale substances are attracting attention for their various functions, and efforts are being made to develop novel nanoscale materials. For example, carbon nanotubes (Carbon Nanotube; CNT) have excellent properties in thermal conductivity, electrical conductance, mechanical strength and the like. Also, by incorporating CNTs with focus on their excellent properties, novel functional films utilizing the properties of CNTs are being actively researched. Examples of such functional films include nanocomposite films.
It is known that in a nanocomposite film containing nanotubes typified by CNTs or nanoparticles, particularly when CNTs are oriented in the film, the variation in film properties owing to the incorporation of the CNTs is remarkable, and electrical property (electric conductivity), mechanical property (elastic modulus) and thermal property (thermal conductivity) are increased compared to those when CNTs are simply mixed. In this regard, “oriented” means that each nanotube typified by a CNT has a diameter of nano meter order and a length of micro meter order, and the nanotubes analogous to one-dimensional substances are lined up in one direction. As for the orientation condition, the nanotubes lined up in one direction are desirably in contact with each other (to become networked).
Because of such excellent properties, nanocomposite films containing oriented nanotubes typified by CNTs or nanoparticles are used in a variety of applications, such as electromagnetic shields, heat dissipation films, transparent electrodes, and anti-static films.
As conventional methods for making CNTs oriented in a film, the following methods have been reported: a technique of producing an electric field between a pair of electrodes to orient the CNTs (e.g., see Japanese Laid-open Patent Publications No. 2004-323342 and No. 2010-027251: PTLs 1 and 2); a technique of applying an AC voltage between a pair of electrodes to produce a thin film consisting of only CNTs between the electrodes (e.g., see Japanese Laid-open Patent Publication No. 2010-091844: PTL 3); a technique for accumulating oriented CNTs onto a plurality of electrodes (e.g., see Japanese Laid-open Patent Publication No. 2006-156389: PTL 4); and a light-emitting element consisting of CNTs grown among a plurality of electrodes, and the electrodes (e.g., see Japanese Laid-open Patent Publication No. 2009-283303: PTL 5).